1. Field of the Invention
The present invention relates generally to a method and system for controlling the secondary air for an internal combustion engine. More particularly, the invention relates to a technology for improving the conversion efficiency of a catalyst by controlling the amount of the secondary air to be supplied to an exhaust passage upstream of the catalyst, in an internal combustion engine to which the catalyst is provided for emission control.
2. Description of the Related Art
Conventionally, there are internal combustion engines which are equipped with catalytic converters for purification of CO, HC and NOx contained in exhaust gas. In such catalytic converters, it is required to maintain an exhaust gas temperature (catalyst temperature) at the inlet of the catalyst to be high enough for promoting a chemical reaction (oxidation and reducing reaction) in the catalyst and thus for assuring the necessary converting ratio.
Therefore, in the prior art, there has been proposed a technology, in which the secondary air (oxygen) is supplied to the exhaust passage upstream of the catalyst for promoting burning (oxidation of CO and HC) in the exhaust system so that the inlet temperature of the catalyst is maintained high enough. (see Japanese Examined Patent Publication No. 53-9663)
When the secondary air is supplied as set forth above, burning of CO and HC contained in the engine exhaust can be promoted upon starting up of the engine where an air/fuel ratio mixture is richer than the theoretical value. By this, the exhaust temperature can be increased to elevate the temperature of the catalyst to an active temperature thereof.
On the other hand, while the catalyst temperature is already at the active temperature, it becomes possible to supply the oxygen by supplying secondary air for promoting an oxidizing reaction between HC and CO while the combustion in the engine is performed using a richer air/fuel ratio mixture than the theoretical value, to cause a lowering of the conversion ratio (oxidizing reaction) of HC and CO in the catalyst due to lack of oxygen.
On the other hand, in the supply of the secondary air as set forth above, it is desired to supply an optimal amount of the secondary air depending upon the air/fuel ratio condition (the concentration of CO and HC). For instance, if an excess amount of the secondary air is supplied upon starting up the engine, an increase in the catalyst temperature can be interfered with by excessive secondary air which is not used for burning of CO and HC. On the other hand, if the amount of the secondary air is too small, burning of CO and HC becomes insufficient and thus makes it impossible to achieve quick elevation of the catalyst temperature as expected for supply of the secondary air.
Furthermore, when the secondary air is supplied after the catalyst reaches the active temperature, the excess amount of the air may cause the lean exhaust air/fuel ratio to lower the conversion ratio of NOx in the catalyst. On the other hand, if the amount of the secondary air is too small, the exhaust air/fuel ratio becomes too rich, making it difficult to achieve satisfactory improvement of the conversion ratio of HC and CO.
As set forth above, the amount of supply of the secondary air is desired to be controlled depending upon the current air/fuel ratio condition. In the control of the secondary air in the prior art, however, the supply of the secondary air is switched between ON and OFF depending on the engine driving condition, or, in the alternative, is controlled on the basis of a predetermined secondary air amount depending upon the engine load and engine speed. Therefore, it has not been possible to control the secondary air precisely depending upon the actual air/fuel ratio, making it difficult to stabilize the conversion ratio of the catalyst at a high level, and thus making it difficult to stably maintain the exhaust characteristics.